One of the operator's main goals is to be able to offer high capacity to their customers in the network. High capacity in terms of number of channels in a cellular network requires in turn a tightening of the frequency plan. That is, more frequencies must be made available in a given area than before. The base station has to handle more carriers at the same site. Conventional systems like TDMA (DAMPS) and GSM require more channels and upcoming systems like the WCDMA instead requires a continuous wide bandwidth. This in turn calls for ultra linear amplifiers.
Linear amplifiers are used to amplify several carriers at the same time, as opposite to amplifying each carrier separately and then add them up in, for example, a hybrid-combiner. Hybrid-combiners such as 90° branch line couplers have the disadvantage that for each doublet of carriers there is a 3 dB power loss. So for example, combining 16 carriers means 4×3=12 dB loss. That means only 6% (=10−12/10) overall power efficiency and, the figure gets worse the more carriers that are added.
A linear power amplifier typically also has an efficiency of about 6% but it keeps relatively constant efficiency as more carriers are added. Moreover, only one amplifier has to be used for all carriers. The main problem with power amplifiers is the linearity of the AM-AM characteristics, whereas hybrid combiners do not suffer from this. Most cellular systems require inter-modulation (IM) products to be in the order of 70 dB down from the carrier. Extensive work has been done to linearize power amplifiers of which feed-forward seems to be the most promising method. Inter-modulation products are simply subtracted at the output of the amplifier by comparing input and output signals of the main amplifier. An error-amplifier adjusts the level of the inter-modulation frequency products (output minus input).
Feed-forward can improve linearity to a certain degree but then it becomes very difficult to achieve the last few dB's necessary for full compliance. A way of further linearizing the amplifier is to pre-distort the input signal to the amplifier and compensate for the non-linearity. There are a number of ways as how to accomplish this. One way is to pre-distort within the feed-forward loop of the MCPA itself. Usually this is done in an analog RF fashion. RF pre-distortion (PD) may also be done outside the full MCPA.
Another way is to implement digital pre-distortion. Digital PD may be used whenever there is a digital combined signal at hand. The introduction of so called software transceivers makes it particularly convenient to extract this signal. On a system-level there would be a digital software transceiver, a broadband digital-to-analog converter (DAC), some RF components and the RF MCPA basically connected to the antenna port. A digital pre-distorter would preferably be placed between the software transceiver and the DAC.
The general idea of pre-distortion is exemplified in FIGS. 1a and 1b. It shows a non-linear MCPA giving IM-spuriouses when only two CW signals are fed to the input connector. So, if the IM is well below the main carrier levels, it would in theory be possible to cancel the IM at the MCPA output by just adding the same CW carriers at IM frequency at the input. But then with a reversed phase in comparison to the output IM. This is the basic idea of pre-distortion, but other signals than CW makes it necessary to actually produce a correct IM-spectrum to be added at the MCPA input port.